Rock bolts are used in rock strata for the purpose of stabilising the strata or rockmass. One type of rock bolt commonly used in hard rock mines is known as a friction bolt. This type of bolt comprises a tube, typically made of steel, that is split longitudinally and which is forced into a bore, drilled into rockmass which is marginally smaller than the diameter of the tube. The tube becomes compressed so that the external surface of the tube engages the internal surface of the bore, anchoring the rock bolt inside the bore by friction forces.
Friction bolts are relatively cheap to manufacture and are easy to use compared with some other types of rock bolts which often require resin or cement to lock them into the bore. However, friction bolts do have a number of drawbacks. One significant drawback is the tendency for friction bolts to disengage from the bore when a sufficiently large force is applied to the bolt.
It is known from U.S. Pat. No. 4,312,604, to weld lands inside the tube, to narrow the internal diameter of the tube at a specific location along the tube, and use oversized, wedge shaped inserts which engage with the lands to expand the tube at that location to provide better engagement of the bolt in the bore at that location. However, the added complexity of the tube of U.S. Pat. No. 4,312,604 increases manufacturing costs, particularly due to the additional components, and the additional step of welding the lands in the correct location inside the split tube.
Recently, more cost effective friction bolts have been developed and are becoming more common in Australian mines exemplified by the friction bolt shown in Australian patent application No 2014215940, by the applicant for the present application, which at least partly addresses the cost and complexity issues of U.S. Pat. No. 4,312,604, by using a floating arrangement for the expander wedges which avoids the need for welding, and thus reduces the manufacturing costs. Other manufacturers have developed their own proprietary rock bolt designs.
A typical friction bolt comprises a generally circular tube defining a longitudinal split, the tube being radially expandable, the bolt having a first leading end for insertion into a bore and a second end defining a head, and expander means for expanding the diameter of the tube at least one location along the tube. The expander elements typically comprising first and second wedge elements arranged so that relative movement of the two wedge elements causes the diameter of the tube to expand at that location. The first wedge element is mounted on a rod which extends along the tube towards the head of the bolt. The second wedge element locates between the rod and the tube. When the rod is rotated the wedge elements move together and cause the diameter of the tube to expand.
One unforeseen problem that has arisen with friction bolts is that in use, when deployed in mines they can become stretched under tension as the rockmass in which they are located can move and become compressed as the material around the strata is mined and removed. In some cases the tension in the rod is such that the rod fractures and fails in tension or shear. The broken rod will then eject from the hole at considerable velocity due to the energy stored in the rod which was under tension. While there are no known instances to date of any injuries caused by the broken rods, these failures are clearly a major potential occupational health and safety issue.
Any discussion of documents, acts, materials, devices, articles or the like which has been included in the present specification is not to be taken as an admission that any or all of these matters form part of the prior art base or were common general knowledge in the field relevant to the present disclosure as it existed before the priority date of each claim of this application.
Throughout this specification the word “comprise”, or variations such as “comprises” or “comprising”, will be understood to imply the inclusion of a stated element, integer or step, or group of elements, integers or steps, but not the exclusion of any other element, integer or step, or group of elements, integers or steps.